1. Field of the Invention
The invention concerns an illumination system, particularly one for lithography, for example, VUV and EUV lithography with wavelengths smaller than or equal to 193 nm, wherein the illumination system has at least one light source, means for producing secondary light sources, which comprises a mirror or lens with raster elements for producing secondary light sources, a real or virtual diaphragm plane, as well as one or more optical elements for imaging the real or virtual diaphragm plane in the exit pupil of the illumination system, and means for producing a predetermined light distribution in the exit pupil of the illumination system, as well as an illumination system for wavelengths of xe2x89xa6193 nm with at least one light source, an object plane, and a plane conjugated to the object plane between light source and object plane.
2. Description of the Prior Art
In order to be able to still further reduce the line widths for electronic components, particularly in the submicron range, it is necessary to reduce the wavelengths of the light used for microlithography.
Lithography with soft x-ray radiation such as has been made known from U.S. Pat. No. 5,339,346 at wavelengths of less than 193 nm.
In addition to illumination according to U.S. Pat. No. 5,339,346, which requires at least four paired mirror facets arranged symmetrically to the source, illumination systems can be provided, which operate, for example, by means of reflective raster element plates for the homogeneous illumination of the ring field of an exposure objective. Such systems have the advantage that the field of an objective is illuminated homogeneously with as few reflections as possible, and further that an illumination of the pupil that is independent of field is assured up to a specific filling degree.
Reflective raster element plates for EUV illumination systems have been made known from U.S. Pat. No. 5,581,605.
The disclosure contents of the above-mentioned publications U.S. Pat. No. 5,339,346 and U.S. Pat. No. 5,581,605 are incorporated to the full extent in the present application.
An increase in the resolution and depth of focus in VUV and EUV illumination systems is possible, if the illumination of the mask can be adjusted each time according to the mask pattern, i.e., according to the reticle to be illuminated. In the prior art, the control of the illumination settings was usually done with a diaphragm, which is arranged downstream to the mirror or to the lens with raster elements. In this respect, for example, refer to:
U.S. Pat. No. 5,526,093
JP-A-10 092,727
JP-A-10 275,771
JP-A-10 062,710
JP-A-09 063,943
An illumination system with an Hg lamp for wavelengths of  greater than 248 nm, in which a scanning diaphragm controls a predetermined quantity that impinges onto an optical integrator is arranged in front of the optical integrator, so that a specific light distribution results in a diaphragm plane, has been made known from JP-A-10-303,123.
U.S. Pat. No. 5,379,090 also shows an illumination system for wavelengths of  greater than 248 nm with an Hg lamp. The system according to U.S. Pat. No. 5,379,090 comprises a variable-focus or zoom objective with which the size of the secondary light source is adjusted. In addition, a diaphragm is used for forming the light distribution on the mask to be illuminated.
Another illumination system for wavelengths of  greater than 248 nm with an Hg lamp has been made known from U.S. Pat. No. 5,363,170. The system according to U.S. Pat. No. 5,363,170 comprises an optical integrator, which is variably illuminated by means of a variable-focus or zoom objective, in order to be able to influence in this way the secondary light sources and thus the illumination in the reticle plane.
DE-A-197 16,794 shows a device for near-type microlithography, which is designed for wavelengths of  greater than 248 nm, with an optical integrator and an optical system for forming a beam. The optical system for forming a beam is arranged in front of the optical integrator and serves for the purpose of forming an elliptical cross-sectional profile, which is adapted to the elliptical profile of the aperture.
U.S. Pat. No. 5,305,054 shows another illumination system for wavelengths of  greater than 248 nm with an optical integrator. In the system according to U.S. Pat. No. 5,305,054, means for splitting the light beam into four light bundles in order to form a quadrupolar illumination are provided in front of the optical integrator.
All of the systems of the prior art for wavelengths of  greater than 248 nm are characterized by the use of purely refractive optical components, and in particular the optical integrators made known from these systems comprise refractive raster element condensers or optical elements with raster elements.
An EUV illumination system with undulator light source has been made known from U.S. Pat. No. 5,896,438, in which an optical integrator can be illuminated with reflective raster elements by means of a scanning mirror. The reflective raster elements of the optical integrator made known from U.S. Pat. No. 5,896,438, have a small aspect ratio of the raster elements and the raster elements are square. The object of the scanning mirror in this illumination device is to increase coherence by an angular scanning of the EUV light beam. To what extent the quality of the mask illumination, i.e., the illumination of the reticle, is influenced by introducing a scanning mirror, has not been made known from U.S. Pat. No. 5,896,438.
The disclosure content of U.S. Pat. No. 5,896,438 is incorporated to the full extent in the disclosure content of the present application.
EUV illumination systems with mirrors comprising a plurality of raster elements, also known as raster elements, which include means for producing a predetermined light distribution in the exit pupil, are known from the post-published document:
WO 99/57732.
It was a disadvantage in the EUV system from the prior art that the control of the light distribution in the diaphragm plane with masks was associated with a considerable loss of light or could not achieve the necessary uniformity in the object plane or reticle plane, since the intermixing or the number of illuminated raster elements was too small.
The object of the invention is to provide an EUV illumination system, with which the disadvantages in the prior art that are indicated above can be avoided. In particular, a system will be provide in which it is possible to control the light distribution in the exit pupil of the illumination system with simultaneous homogeneous illumination of the object plane or the reticle plane for any desired VUV and EUV sources.
In a first embodiment of the invention, in an EUV illumination system, it is provided that a number of raster elements, also know as grid elements or honeycombs, will be illuminated by the means for producing a defined illumination distribution in the exit pupil, so that a predetermined uniformity is achieved in the object plane.
In an alternative embodiment of the invention, in an illumination system that has a plane conjugated to the object plane, it is provided to arrange means for producing a predetermined light distribution in the vicinity of or in the plane conjugated to the object plane.
The following are currently discussed as light sources for EUV illumination systems:
Laser plasma sources
Pinch plasma sources
Synchrotron radiation sources.
In laser-plasma sources, an intensive laser beam is focused on a target (solid, gas jet, droplets). By excitation, the target is heated so intensely that a plasma is formed. This plasma emits EUV radiation.
Typical laser plasma sources have a spherical irradiation, i.e., a beaming angular range of 4xcfx80 as well as a diameter of 50 xcexcm-200 xcexcm.
In pinch plasma sources, the plasma is produced by means of electrical excitation
Pinch plasma sources can be described as volume radiators (Ø=1.00 mm, which irradiate in 4 xcfx80, wherein the radiation characteristic is given by the source geometry.
In synchrotron radiation sources, three types of sources can be currently distinguished:
(1) bending magnets
(2) wigglers
(3) undulators.
In bending magnet sources, the electrons are deflected by a bending magnet and emit photon radiation.
Wiggler sources comprise a so-called wiggler, which is made of a plurality of magnet pairs arranged in rows of alternating polarity, for deflecting electrons or an electron beam. If an electron passes through a wiggler, then the electron is subjected to a periodic, vertical magnetic field; the electron oscillates correspondingly in the horizontal plane. Wigglers are further characterized by the fact that coherence effects do not occur. The synchrotron radiation produced by means of a wiggler is similar to that of a bending magnet and radiates into a horizontal solid angle element. In contrast to the bending magnet, the wiggler has a flux enhanced by the number of poles that it has.
There is no clear dividing line between wiggler sources and undulator sources
In undulator sources, the electrons in the undulator are subjected to a magnetic field with a shorter period and smaller magnetic field of the deflecting poles than in the wiggler, so that interference effects of synchrotron radiation occur. The synchrotron radiation has a discontinuous spectrum due to the interference effects and irradiates both horizontally as well as vertically in a small solid-angle element; i.e., the radiation is strongly directional.
With respect to the construction of EUV illumination systems in principal, reference is made to the pending application EP 99 106348.8, filed on Mar. 2, 1999, with the title xe2x80x9cillumination system, particularly for EUW lithographyxe2x80x9d; U.S. Ser. No. 09/305,017 filed on May 4, 1999, with the title xe2x80x9cIllumination system particularly for EUV lithographyxe2x80x9d; and WO 99/57732, filed on May 4, 1999, with the title xe2x80x9cIllumination system, particularly for EUV lithographyxe2x80x9d of the Applicant, the disclosure content of which is incorporated in its entirety in the present application.
It is provided in an advantageous embodiment of the invention that the means for producing the light distribution comprise a scanning mirror, such as, for example, the one in U.S. Pat. No. 5,896,438. Preferably, such a scanning mirror can be controlled.
The use of a scanning mirror has the advantage of a loss-free control of the illumination distribution. In particular, scanning mirrors are used for light sources with a small Entendu, for example, undulator light sources. In such systems scanning mirrors are used for increasing the Entendu.
In a system without a plane conjugated to the object plane between light source and object plane, illumination can be produced appropriately by a corresponding control of the scanning movement of the mirror with raster elements in order to produce secondary light sources. Alternatively to a scanning mirror, an optical system with a zoom effect can be arranged in front of the mirror with raster elements. With such an optical system, variable focal lengths can be illuminated by displacement of individual optical components if the image plane, which is here the plane in which the mirror with raster elements is situated, is held constant.
In another advantageous form of embodiment of the invention for systems without a conjugated plane, relative to the object plane, for producing a light distribution of predetermined size, the entire collector unit comprising of optical elements of positive and/or negative refractive power, for example, of divergent mirrors or convergent lenses can be changed in total.
With such an embodiment it is possible to illuminate a larger or smaller circle on the mirror with raster elements, which also can be denoted as field raster element plate.
The means for producing a specific light distribution in another embodiment of the invention may comprise a mirror or a lens with deformable surface, which is used both for systems with a plane conjugated to the object plane as well as for those without this plane.
Mirrors with deformable surface have been made known, for example, from JP-A-51 0097 or from U.S. Pat. No. 5,825,844, wherein the disclosure content of these publications is fully incorporated into the present application.
In JP-A-51 00097, the surface deformation is stimulated by means of piezoelectric crystals. Therefore, a plurality of manipulators is introduced on the underside of the mirror. An electromagnetic excitation has also been made known from U.S. Pat. No. 5,825,844, in addition to excitation with piezoelectric crystals.
The deformable mirror for producing a predetermined light distribution in the exit pupil can be arranged, for example, in the collector unit; in illumination systems with intermediate image, i.e., with a plane conjugated to the object plane or reticle plane, the deformable mirror may also be arranged in this plane.
There are a multiple number of possibilities for light distribution in the exit pupil. In a first embodiment of the invention, it is provided that the exit pupil is illuminated circularly. Alternative to a circular illumination, a ring-shaped illumination or a quadrupole illumination can be provided.
In order to assure the uniformity of field illumination in systems in which the field has a high aspect ratio, it is advantageously provided that the raster elements of the mirror or lens for producing secondary light sources comprise field raster elements with an aspect ratio that is smaller than the aspect ration of the field and are, for example, of cylindrical and/or toroidal shape In this way, the number of incompletely illuminated field raster elements is reduced. With respect to the effect of such systems, reference is made to the pending application DE-A-199 31 848.4, filed on Jul. 9, 1999, with the title: xe2x80x9cComponents with anamorphotic effect for reducing the raster element aspect ratio in EUV illumination systemsxe2x80x9d, the disclosure content of which is incorporated to the full extent in the present application.
The uniformity, i.e., variation of the intensity distribution within a pregiven field in the object plane or reticle plane, which can be achieved by means of the device according to the invention, is  less than 10%, preferably  less than 5%, and most preferably  less than 1%. Whereas in stepper systems, the uniformity of the intensity distribution is considered within a pregiven field, in the case of scanner systems, the uniformity of the scanning energy must be given for each field point perpendicular to the scanning direction.
In addition to the illumination system, the invention also makes available an EUV projection system with such an illumination system, as well as a method for producing microelectronic components.